If the BJU is going fine, you can certainly continue with it until you get to the end of the materials you purchased. I will say that the teacher presentation part in BJU is time consuming, especially as you get into the upper levels (although it is well-done). I would weigh whether you think you will be able to continue with the needed presentation that BJU requires as you go through the years as this will also raise the level of time that your overall school day takes.
I realize there are many good math programs out there, and BJU is one of them. I just mentioned the concerns I listed above because they are the same concerns we ran into when weighing whether to stay with BJU with my oldest son (after we'd done 'K' math with BJU). We did return to BJU at a later time with my oldest son (for part of grade 4), but I wasn't able to give the time to the lesson presentation and preparation that was required, and so it ended up to be more of a workbook for us (which is not how it was intended to be used).
With that being said, we have used Singapore all the way with my second son, and I find it is easier to use a program that does not require presentation by me but is an added benefit that I can easily add-in if needed. Once we got to level 3A/3B, my son was ready for me to just go over the problems in the textbook with him, and then he was eager to go do the workbook part on his own. Just some food for thought.
Most moms say it is also easier to switch to Singapore earlier than it is to switch later.
Blessings,
Carrie